Mild winters raise risk of flu epidemics

Arizona State University mathematician Sherry Towers has found links between a warm winters with little flu to epidemics the following year. Credit: Arizona State University

In 2012/13 the US flu season started especially early for two strains for the first time since the government started tracking it in 1997. Sherry Towers at Arizona State University has put this down to the unusually warm winter the country saw in 2011/2012, after showing such a link can be seen in previous years. She hopes that her findings can help health services prepare as winters get yet milder with continuing climate change.

“Until now, it had not been noticed that the dynamics of the current season depend not only on the temperature of the current season and vaccine match, but also on what had occurred the year before,” Sherry told me. “If there has been a mild flu season during a mild winter, public health authorities know several months in advance that a severe season with early onset is much more likely to occur the next season. This allows them to expedite the manufacture and distribution of vaccines to the population.”

As a mathematician looking at how climate affects the spread of infectious diseases, Sherry follows influenza data collected by the US Centres for Disease Control (CDC) closely. The CDC tracks the various influenza virus types in circulation. These include letter and number combinations you might have heard, like H1N1 and H3N2, which together are classed as influenza A, plus the single type of influenza B virus. Scientists had previously shown that high temperatures reduce transmission of the virus, which alone would make mild flu seasons more common in warm winters. Though this suggests less of a threat from flu in a warmer world, this season’s data made her wonder if there could be a downside.

Defences down

Though warmer winters reduce the spread of the flu virus (shown here), that means people have lower immunity the following season, putting them at higher risk of infection. Credit: hitthatswitch via Flickr

“I noticed this year was extraordinarily unusual in that both the influenza B and H3N2 epidemics began very early,” Sherry said. “Since surveillance began, two strains at the same time have never done that. Last year was a record mild influenza season, which is not surprising given that it was a very warm winter. I wondered if what was happening this year could have been caused by that. However, a conclusion that there might be a possible link between severe influenza seasons following mild winters is a lot more compelling if it can be shown that the pattern exists in different geographic regions, and also for the different types of influenza.”

Sherry and a group of other researchers from Arizona State University and Northeastern Illinois University in Chicago teamed up to test her idea. They looked at the relationship of climate to epidemic patterns in H1N1, H3N2, and B in 10 geographic regions over 16 years recorded by the CDC. They found a link between climate in the previous flu season to each year’s epidemic dynamics that passed the key ‘statistical significance’ test.

The link is a consequence of the need to be exposed to flu to build up an immune defence if you don’t get a vaccine. “65% of the population doesn’t get the flu shot,” Sherry says. “If many people are not infected with flu, and they don’t get immunized, they have little immunity to flu going in to the next season. An unnaturally large fraction of vulnerable people in the population after the mild flu season can then result in an early and severe onset in the next season.”

The researchers found that the top approximately 15% of warmest winters showed a ‘significant’ link to early and severe epidemics the next season. Almost three quarters of the following epidemics were more severe, growing 40% faster and occurring 11 days earlier than average. That’s even taking into account the expectation that the warmer winters climate change brings would make it harder for the virus to get from one person to another. “Intuitively, it seems like things might balance out, but in reality that is not the case,” Sherry explained. “When a flu season has a very early onset much of the population has not had a chance to get vaccinated, and flu seasons are significantly worse if vaccinations occur during the flu season rather than before it.”

CDC flu data, with lines of best fit drawn in red through the scatter of points. Flu seasons start and peak earlier and grow faster the hotter the previous winter, and the milder the previous flu season was. Credit: PLoS Currents Influenza via Creative Commons license, see below for full citation

[…] covering this year has shown the wide range of impacts climate change is having, and will have. It raises the risk of flu epidemics. It is altering weather patterns, bringing extreme cold to Europe and unusual warmth to Greenland. […]

[…] Warming is already showing that it can kill many species, like the seaweed dying off during a record-breaking heat wave in Australia in 2011. That puts us at risk of undermining ecosystem functions like pollination, which is critical for food production, and resilience from losing key species in food chains. Higher temperatures can also directly harm our health, for example raising the risk of flu epidemics. […]